Plate blocks, also known as plate-number blocks, refer to stamps that are still attached to their original sheet and including the serial number of the printing plate in the sheet’s margin or selvage. The format of the plate-block numbers depends on when the stamps were printed, and who did the printing. Prior to 1894, when the U.S. Bureau of Engraving and Printing (BEP) began printing stamps, private companies were contracted for this task....Continue Reading